Device with moisture filter

ABSTRACT

A chamber in the interior of a housing communicates with the environment of the housing by means of a gas esxchange path, wherein a filter element with a hydrophobic and/or hydrophobicized, nanoporous material is placed in the gas exchange path. The chamber serves, e.g. to accomodate electronic circuitry, or it is chamed as a reference pressure chamber of a relative pressure sensor. The relative pressure sensor according to the present invention for capturing a measured pressure with respect to a reference pressure comprises a reference pressure path  23  which extends between a surface which can be exposed to the reference pressure and an opening in a reference pressure chamber  22,  a filter element  30  which is arranged in the reference pressure path comprising a hydrophobic or hydrophobicized, nanoporous material. The filter element allows pressure compensation on account of its permeability to N 2  and O 2 , while water molecules are selectively blocked. The nanoporous material preferably comprises an inorganic material, in particular Al 2 O 3 , TiO 2  or SiO 2 . The nanoporous material is preferably hydrophobic and/or can be impregnated with a hydrophobic layer, preferably a silane, in order to optimize the hydrophobic properties.

[0001] The invention generally relates to housings or devices withmoisture sensitive components, such as e.g. housings with moisturesensitive electronic circuitry or measuring devices with moisturesensitive sensors. This group of devices especially comprises relativepressure sensors.

[0002] Relative pressure sensors can be used to measure pressures ofmedia, for example of liquids, gases or vapors, with respect to thecurrently prevailing atmospheric or ambient pressure, this atmosphericor ambient pressure therefore serving as a reference pressure. In thiscontext, the humidity of the reference air has proven to be a problem,since the humidity can penetrate into the sensor via a referencepressure line and can condense out at temperatures which lie below thedew point. Therefore, there have been extensive efforts to prevent themoisture from penetrating into the sensor.

[0003] By way of example, Japanese patent application No. 07110364 hasdisclosed a capacitive relative pressure sensor with a base body and adiaphragm which is connected, along its edge region, in a pressure-tightmanner to the base body, so as to form a reference pressure chamber. Thereference air is introduced into the reference pressure chamber througha reference pressure line and a bore. In the reference pressure linethere is a water absorber which is intended to dry the reference air.

[0004] This solution is unsatisfactory to the extent that the moisturecontent in the absorber is enriched. When the absorber is saturated, orwhen the temperature rises and some of the bound water can be desorbedagain, moisture passes into the sensor.

[0005] The European patent application which bears the publicationnumber 974 825 A2 follows a different approach. The structure of therelative pressure sensor is generally as described above, but ahydrophobic filter element is used instead of the absorber, this elementbeing held at a temperature which the temperature inside the sensornever falls below and is preferably significantly colder than thetemperatures in the interior of the sensor. In this way, moisturecondenses out at the filter element when the filter temperature fallsbelow the dew point of the warmer ambient air. Therefore, the air whichenters the interior of the sensor only has a water content whichcorresponds to a relative atmospheric humidity of 100% at the filtertemperature. However, since the temperature inside the sensor neverfalls below the filter temperature and is generally in fact above thefilter temperature, there is no possibility of the moisture condensingin the interior of the sensor, since the dew point is not reached.

[0006] The required cooling of the filter element is achieved, forexample in sensors for the food industry, by keeping the filter elementin thermal contact with the cold process medium via the sensor housing.The device described is advantageous to the extent that the saturationproblems which arise with an absorber do not occur.

[0007] On the other hand, the need to control the temperature of thefilter element entails significant design limitations which areunacceptable for certain applications.

[0008] Moreover, the user is subject to restrictions in terms ofoperation and maintenance of the pressure-measuring device. By way ofexample, if the filter element were to be heated, during cleaningoperations, to temperatures which lie above the normal operatingtemperature in the interior of the sensor, it would be possible for airwith a high level of humidity to penetrate into the sensor, and thismoisture could then condense at normal operating temperature.

[0009] For similar reasons, the above filter elements or absorbers areunsuitable for other devices with moisture sensitive components whichrequire a gas exchange with the ambient for the purpose of cooling orpressure adjustment. This especially applies, if changes of the ambienttemperature with associated changes of the reative humidity must beexpected. Such moisture sensitive components may be, for instance,electronic circuits.

[0010] The present invention is therefore based on the object ofproviding a device with a filter element which overcomes the problemsdescribed.

[0011] According to the invention, the object is achieved by therelative pressure sensor according to the independent patent claim 1,and by the device according to independent patent claim 8. Furtheradvantages and aspects of the invention are given in the dependentclaims, the description and the drawings.

[0012] The device comprises a housing which defines a chamber in itsinterior which comprises at least one aperture through which the chamberis in fluid communication with the environment of the housing by meansof a gas exchange path; and a filter element disposed in the gasexchange path, said the filter element comprising a hydrophobic and/orhydrophobicized, nanoporous material.

[0013] An especially preferred embodiment the chamber comprises an inletaperture and an outlet aperture, through which the chamber is in fluidcommunication with the environment of the chamber by means of respectivegas exchange paths, wherein a respective filter element is provided inboth gas exchange paths, said filterelements comprising a hydrophobicand/or hydrophobicized, nanoporous material

[0014] The device is especially suitble as housing for electroniccircuitry. Optionally, an airflow can be guided through the chamber bymeans of a conventional ventilator, therby enabling an effective heatexchange.

[0015] The relative pressure sensor according to the invention forcapturing a measured pressure with respect to a reference pressurecomprises:

[0016] a sensor element having

[0017] a base body and

[0018] a measurement diaphragm which, along its edge region, isconnected in a pressure-tight manner to the base body so as to form areference pressure chamber,

[0019] the measurement diaphragm having a first diaphragm surface, whichfaces away from the reference pressure chamber and can be exposed to themeasured pressure, and a second diaphragm surface, which faces thereference pressure chamber;

[0020] a reference pressure path which extends between a surface whichcan be exposed to the reference pressure and an opening in the referencepressure chamber, with the result that the second diaphragm surface canbe exposed to the reference pressure; and

[0021] a filter element which is arranged in the reference pressurepath;

[0022] wherein

[0023] the filter element comprises a hydrophobic and/orhydrophobicized, nanoporous material.

[0024] The nanoporous material is preferably arranged as a layer on aporous support material or is embedded in the matrix of a porous supportmaterial, the support material serving in particular to ensure therequired mechanical stability.

[0025] The nanoporous material preferably comprises an inorganicmaterial, in particular a ceramic material, Al₂O₃ or TiO₂ beingpreferred. ZrO2, SiO₂, aluminosilicates, aluminum beryllium silicates;apatite, cordierite, mullite, zeolite, SiC and Si₃N₄, carbon, VycorGlass and their mixtures are in principle also suitable.

[0026] In this context, the term “nanoporous” refers to a pore sizedistribution whose maximum, based on the pore frequency, lies at a porediameter of less than 4 nm, preferably less than 2.5 nm, more preferablybetween 0.4 and 2 nm, even more preferably between 0.5 and 1.5 nm, andparticularly preferably between 0.7 and 1 nm. In a particularlypreferred exemplary embodiment, the distribution maximum isapproximately 0.9 nm.

[0027] The preferred production process using the sol gel process can beused to achieve a sufficiently narrow pore size distribution whichensures a uniform filter action. The maximum pore diameter shouldpreferably be no more than 10 nm, more preferably no more than 5 nm,even more preferably no more than 2 nm.

[0028] The terms microporous and mesoporous are also customarily used infiltration technology to describe layers with pore sizes in thenanometer range. According to this technology, what are known asmesoporous layers have pore diameters of between 2 nm and 50 nm, whilewhat are known as microporous layers have pore diameters of less than 2nm. In the context of these definitions, the nanoporous material used inthe invention is to be classified in the microporous range or at thelower end of the mesoporous range. In the text which follows, the term“nanoporous” will continue to be used in connection with the materialsused according to the invention.

[0029] The layer thickness of the nanoporous material may in each casebe adapted to the desired filter action, which is dependent on the poresize distribution and the hydrophobic properties. The layer must besufficiently thick, so that there are no defects in the material whichwould endanger the filter action.

[0030] On the other hand, it is recommended for the nanoporous layer tobe as thin as possible, in order to minimize the flow resistance for thegases which are to be allowed to pass through, for example N₂ or O₂.Finally, the surface of the filter element is to be designed in such away that, with the flow resistance per unit area which is present onaccount of the required filter action, it is possible to achievesufficiently rapid pressure compensation through the filter element inthe event of pressure pulses.

[0031] Typical conductivities for gaseous media in the case of filterelements with nanoporous layers are approximately 20 to 800 mm³/(cm²sbar), preferably 20 to 200 mm³/(cm²s bar).

[0032] The layer thickness of the nanoporous material is preferably atleast 20 nm, more preferably at least 40 nm, and particularly preferablyat least 80 nm. The layer thickness of the nanoporous material ispreferably no more than 4 μm. more preferably no more than 2 μm, evenmore preferably no more than 1 μm, and particularly preferably no morethan 0.5 μm.

[0033] Particularly in the case of applications which involveconsiderable temperature fluctuations, the support material for thenanoporous layer should preferably have the same thermal expansionbehavior as the filter material, in order to minimize the mechanicalstress on the filter material. However, the importance of thisrequirement decreases as the layer thickness of the nanoporous materialfalls.

[0034] Layer systems with 2, 3 or more chemically and/or morphologicallydifferent layers are also suitable for implementing the invention. Inthis context, preference is given to layer systems in which layers withstepped pore sizes follow one another, the layer systems having at leastone layer of nanoporous material.

[0035] In a preferred group of filter elements, these elements have alayer structure with a nanoporous layer of TiO₂ with a mean pore size ofapproximately 0.7 to 1.2 nm, preferably approximately 0.9 nm, thenanoporous layer having a mean thickness of between 30 and 100 nm,preferably between 40 and 70 nm, particularly preferably between 45 and55 nm. The nanoporous layer described is applied to a ceramic middlelayer, preferably of TiO₂, with a mean pore size of between 3 and 10 nm,preferably between 4 and 7 nm, particularly preferably between 5 and 6nm, and with a mean thickness of 300-1000 nm, preferably 400-800 nm,particularly preferably about 450-550 nm. The middle layer is in turnapplied to a series of support layers with increasing pore sizes andincreasing thicknesses, these layers preferably containing Al₂O₃. Thesupport layer which has the largest pores, on top of which the supportlayers with finer pores are formed, preferably has a mean pore size of afew μm, preferably between 2 and 10 μm, particularly preferably between2.5 and 5 μm, and a thickness of a few 100 μm to about 2 mm.

[0036] The layers which are active in separation are preferably producedusing a sol gel process, as described, for example, by Larbot et al. inInternational Journal of High Technology Ceramics 3 (1987), pages143-151.

[0037] With the sol gel process, the pore size can be controlled veryaccurately by means of the hydrolysis and condensation conditions.Moreover, the firing temperature is to be adapted to the desired poresize, as explained by Larbot et al.

[0038] Suitable diaphragm materials are also commercially available fromthe Hermsdorfer Institut fur Technische Keramik or Inocermic GmbH. Othersuitable materials are marketed under the trade name KEMIHOFA®.

[0039] The hydrophobic properties of the filter material can be improvedby suitable coatings. For this purpose, the diaphragm may preferably beimpregnated with an organic solvent which contains a hydrophobicsubstance in solution. Although the concentration of the substance isnot critical, concentrations of between 0.5-20% by weight, preferablyapproximately 0.5-10% by weight, particularly preferably 0.5-5% byweight, have proven suitable for the hydrophobicizing.

[0040] In principle, any desired hydrophobic substances, such as waxes,aliphatic and aromatic hydrocarbons, silicones and silicone resins, aresuitable. Preference is given, inter alia, to organosilanes, inparticular silanes of the formula R_(y)—Si—X_(4-y), where R denotes ahydrophobic radical, X denotes a hydrolyzable group and 1≦y≦3. Methylsilanes, phenyl silanes, octadecyltrichlorosilane,di-(dodecyl)-difluorosilanes and other fluorine-containing silanes areparticularly preferred. R_(y)—Zr—X_(4-y) and R_(y)—Ti—X_(4-y) are alsosuitable.

[0041] The impregnation is promoted by an open porosity with pore widthswhich allow the carrier medium and the hydrophobicizing agent topenetrate. Layers which have not completely solidified and the structureof which swells during the impregnation step are also suitable.

[0042] Further details about hydrophobic coatings have been published,for example, in the British patent application GB 2 014 868 A.

[0043] Moreover, hydrophobic material can also be deposited on ananoporous layer by vapor deposition, in order to hydrophobicize thislayer. Vapor deposition is particularly suitable if solvents impair thecoating operation.

[0044] As an alternative to the subsequent impregnation of the filtermaterial with a hydrophobic solution, it is also possible for ahydrophobic substance to be admixed with the starting materials.Examples of such substances are organic/inorganic hybrid materials, suchas ormocers (a contraction of organic modified ceramics). Examples ofsuitable starting materials for this purpose are tetraethylorthosilaneand methyltriethoxysilane.

[0045] Nanoporous layers based on pyrolyzed carbon or based on colloidalorganic, hydrophobic particles do not require any additionalhydrophobicizing, since they are already inherently sufficientlyhydrophobic.

[0046] A filter element which has been designed according to theprinciples described above ensures that the water contained in the airis retained by the filter element, even at temperatures above the dewpoint, without impairing the pressure compensation between the ambientair and the reference pressure chamber. In this way, the water contentof the air in the reference pressure chamber can be kept so low that therelative atmospheric humidity in the reference pressure chamber remainsbelow 100% for a prolonged period even at low temperatures which liebelow the dew point of the environment of the sensor, so thatcondensation of water can be avoided.

[0047] Although the filter element can be arranged at any desiredposition on the reference pressure path, it is preferable for the filterelement to be arranged in the vicinity of the surface which can beexposed to the reference pressure or for the filter element to beintegrated into this surface.

[0048] A relative pressure sensor usually comprises a housing in whichthe sensor element is arranged, and the surface which can be exposed tothe relative pressure is in this case preferably a surface of thehousing.

[0049] Electronic circuits for generating a measurement signal areusually arranged in a section of the housing of the relative pressuresensor.

[0050] The invention will now be described with reference to theappended drawings, in which:

[0051]FIG. 1 shows a longitudinal section through a relative pressuresensor according to the invention.

[0052]FIG. 2 shows the frequency distribution of the pore diameters forthe nanoporous material of a preferred filter element;

[0053]FIG. 3 shows the step response of a relative pressure sensor withfilter according to the invention to a pressure pulse compared to thestep response of a relative pressure sensor without is filter, and alsosuperimposes the two step response curves; and

[0054]FIG. 4 shows a housing according to the present invention forelectronic circuitry

[0055] The relative pressure sensor according to the invention comprisesa housing 10, in the interior of which a sensor element 20 which ispreferably of cylindrical structure is arranged.

[0056] The sensor element comprises a base body 26, which is connectedin a pressure-tight manner to a measurement diaphragm 21, a referencepressure chamber 22 being formed between the measurement diaphragm. Afirst diaphragm surface faces away from the base body 26 and can beexposed to a measurement pressure which can be introduced into a cavity12 in the housing through a connection 11. The cavity is delimited bythe measurement diaphragm 21, an annular seal 13 preferably beingclamped axially between a stop face of the housing and the measurementdiaphragm 21, in order to seal the cavity.

[0057] The pressure-dependent deformation of the measurement diaphragm21 is detected by suitable means, for example capacitively, resistivelyor inductively, and is converted into a measurement signal which isremoved from the housing via a line 27. Details of the way in whichthese measurement principles are implemented are common knowledge to theperson skilled in the art and require no further explanation here.

[0058] The second diaphragm surface, which faces the base body 26, ofthe measurement diaphragm 21 can be exposed to the reference pressure,which is introduced into the reference pressure chamber 22 via areference pressure line 23.

[0059] The reference pressure line 23 is preferably guided radially outof the housing 10 and opens out in a filter chamber 24 which has anopening 25 leading to ambient air. A diaphragm-like filter element 30,which takes up the entire cross-sectional area of the filter chamber 24,is arranged in the filter chamber 24. In its edge region, the filterelement is connected in a gastight manner to the filter chamber, so thatall the gas has to be transferred through the surface of thediaphragm-like filter element.

[0060] The filter chamber preferably has a cover with a gas inlet, inorder to protect the filter element from mechanical damage. Althoughother arrangements of the filter element are conceivable, it is inprinciple advisable for the filter element to be designed so that it isprotected from mechanical loads. Equally, the filter element is to beprotected from aggressive reagents, if the relative pressure sensoraccording to the invention is to be used in the vicinity of suchsubstances.

[0061] The filter element has a nanoporous material which is impregnatedwith a hydrophobic organosilane.

[0062] The filter element of the exemplary embodiment in this case hasthe layer structure presented in Table 1 below. TABLE 1 Material Layerthickness Mean pore size TiO₂ 0.05 μm 0.9 nm   TiO₂  0.5 μm 5 nm Al₂O₃10-20 μm 60 nm  Al₂O₃ 10-20 μm 200 nm  Al₂O₃  100 μm 1 μm Al₂O₃ Support400-600 μm 3 μm

[0063] The pore size distribution of the filter element with a mean poresize of 0.9 nm is illustrated in FIG. 2. On account of the very narrowpore size distribution, a homogeneous filter action is to be expectedfor the entire filter element.

[0064] To check whether a filter element impairs the start-upperformance of the relative pressure sensor according to the invention,a relative pressure sensor with a nominal pressure range of 400 mbar wasexposed to an overload pressure of 6 bar. The step response after thepressure had been relieved as quickly as possible was measured in eachcase with a filter element and without a filter element. In thisembodiment, the relative pressure sensor had an internal volume ofapproximately 20 mm³ without the application of pressure. The internalvolume when pressure was applied with an overload was approximately 7mm³, meaning that a gas volume of approximately 13 mm³ at standardpressure had to flow through the filter element in order to reach theequilibrium state after the pressure relief. Representative results ofthe measurements are shown in FIG. 3, in which the top curve in the topdiagram shows the step response with a filter and the bottom curve inthe top diagram shows the step response without a filter. The bottomdiagram superimposes the two curves, the two curves runningsubstantially identically. It follows from this that the step responseof the filter element is practically unimpaired.

[0065] The start-up time with and without filter element was 210 ms, theoutput signal after this start-up time being in an error band of ±0.25%.It was impossible to detect any lag effects over a measurement time ofapproximately 10 sec.

[0066] Consequently, the measurement accuracy and the start-up time ofthe relative pressure sensor in response to pressure fluctuations arenot impaired in any way by the filter element.

[0067] During the production of the relative pressure sensor accordingto the invention, it should be ensured that the reference pressurechamber 24 is absolutely dry, since moisture contained in the referencepressure chamber would be unable to escape on account of the filterfunction.

[0068] The sensor element of the relative pressure sensor according tothe invention may in particular be a capacitive, resistive or inductivesensor element, but the invention can be implemented with any sensorelement irrespective of the measurement principle selected.

[0069] In principle, any desired arrangements are possible for therelative pressure sensor. For example, the first diaphragm surface ofthe measurement diaphragm may be brought into direct contact with themedium to be measured, or alternatively it is possible to provide aseparating body with a separating diaphragm, in which case theseparating diaphragm comes into contact with the medium to be measuredand the pressure which is present at the measurement diaphragm ishydraulically transferred to the first diaphragm surface of themeasurement diaphragm. The measures required for this purpose are wellknown to the person skilled in the art and do not require extensiveexplanation here.

[0070] Finally, FIG. 4 shows a housing for electronic circuitry 40 whichdefines a chamber 41 in its interior. The chamber comprises a firstaperture 42 and a second aperture 43, through which the chambercommunicates with the environment of the housing. Filter elements 44, 45are placed in the first aperture 42 and the second aperture 43,respectively, wherein said filter elements comprise a hydrophobic and/orhydrophobicized, nanoporous membrane. Regarding the details of thefilter elements, reference is made to the above description of thefilter elements for relative pressure sensors according to the presentinvention. The filter elements described in that context are suitablefor the housing for electronic circuitry according to the presentinvention without any restriction.

[0071] A ventilator may be provided optionally to increase the gas flowthrough the chamber 41. This might be necessary if the heat dissipatedfrom electronic circuitry (not shown) cannot be removed sufficiently bythermal convection.

[0072] Further aspects regarding the dimensions of the filter elementsfor a given volume of the housing or a required cooling rate are easilyderivable for a person skilled in the art, without any further detaileddiscussion.

1. A relative pressure sensor for capturing a measured pressure with respect to a reference pressure, comprising: a sensor element having a base body and a measurement diaphragm which, along its edge region, is connected in a pressure-tight manner to the base body so as to form a reference pressure chamber, the measurement diaphragm having a first diaphragm surface, which faces away from the reference pressure chamber and can be exposed to the measured pressure, and a second diaphragm surface, which faces the reference pressure chamber; a reference pressure path which extends between a surface which can be exposed to the reference pressure and an opening in the reference pressure chamber, with the result that the second diaphragm surface can be exposed to the reference pressure; and a filter element which is arranged in the reference pressure path; wherein the filter element comprises hydrophobic or hydrophobicized, nanoporous material.
 2. The relative pressure sensor as claimed in claim 1, in which the nanoporous material is arranged as a layer on a support material or in the matrix of a porous support material.
 3. The relative pressure sensor as claimed in claim 1 or 2, in which the nanoporous material contains one of the substances Al₂O₃, TiO₂, ZrO₂, B₂03, CeO₂, mullite, zeolite, silicates (aluminosilicates, aluminum beryllium silicates, apatite, cordierite), phosphates, SiC or Si₃N₄, carbon, Vycor Glass or mixtures of these substances.
 4. The relative pressure sensor as claimed in claim 1 or 2, in which the nanoporous material comprises an ormocer or an inorganic polymer, in particular silicone resin, polycarbosilanes or polycarbosilazanes.
 5. The relative pressure sensor as claimed in one of claims 1 to 4, in which the nanoporous material has a mean pore diameter of less than 4 nm, preferably less than 2.5 nm, more preferably between 0.4 and 2 nm, even more preferably between 0.5 and 1.5 nm, and particularly preferably between 0.7 and 1 nm.
 6. The relative pressure sensor as claimed in one of the preceding claims, in which the nanoporous material is laid with a hydrophobic layer.
 7. The relative pressure sensor as claimed in claim 6, in which the hydrophobic layer contains at least one organosilane.
 8. A device, comprising a housing (40) which defines a chamber (41) in its interior which comprises at least one aperture (42, 43) through which the chamber is in fluid communication with the environment of the housing (40) by means of a gas exchange path: and at least one filter element (44, 45) disposed in the gas exchange path, said filter element comprising a hydrophobic and/or hydrophobicized, nanoporous material. -
 9. The device of claim 8, -wherein the chamber (41) comprises an inlet aperture (42) and an outlet aperture (43), wherein filter elements (44, 45) are disposed in both gas exchange paths, respectively, said filterelements (44, 45) comprising a hydrophobic and/or hydrophobicized, nanoporous material.
 10. The device according to claim 9, further comprising a ventilator to effect an air flow through the inlet aperture 44 and the outlet aperture
 45. 